This invention generally relates to control rod guide tubes and more particularly relates to a control rod guide tube adapted to reduce vibration of a plurality of control rods disposed therein, which control rod guide tube is suitable for use in pressurized water nuclear reactor pressure vessels.
Although devices for reducing vibration of control rods are known in the prior art, it has been observed that these prior art devices have certain limitations associated with them. However, before these limitations can be appreciated, some background is necessary as to the structure and operation of a typical pressurized water nuclear reactor pressure vessel and its associated control rod guide tubes.
In this regard, a nuclear reactor is a device for producing heat by controlled fission of neutron-producing fissionable material contained in fuel assemblies. A plurality of the fuel assemblies are grouped in a sealed reactor pressure vessel to define a nuclear reactor core therein. Pressurized liquid moderator coolant is caused to circulate through the pressure vessel and thus through the fuel assemblies for assisting in the fission process and for removing the heat produced by fission of the fissionable material contained in the fuel assemblies.
However, the neutrons produced by the fission process in the reactor core must be suitably controlled for safety reasons. Therefore, disposed in the pressure vessel are a plurality of control rod guide tubes, each control rod guide tube being aligned with its associated fuel assembly. Each control rod guide tube houses a plurality of movable control rods belonging to a control rod cluster assembly. Each control rod cluster assembly is slidably movable so that the control rods belonging to the control rod cluster assembly are capable of being slidably inserted into and withdrawn from each fuel assembly to control the fission process therein. Moreover, disposed in each guide tube are a plurality of guide plates having bores therethrough for slidably receiving respective ones of the control rods so that the control rods are suitably guided and laterally supported by the guide plates.
As the previously mentioned liquid moderator coolant circulates through each fuel assembly, it continues its upwardly flow path and enters the guide tube associated with that fuel assembly. As the liquid moderator coolant exits the guide tubes, it flows to a heat exchanger or steam generator for providing steam by means well known in the art.
In addition, the control rods themselves generate heat due to the nuclear transformations occurring therein as they absorb the fission neutrons that are produced by the fission process. Therefore, the control rods are cooled by the liquid moderator coolant to remove the heat generated by the control rods. In this regard, as the coolant enters the guide tube it suitably cools the control rods.
As the coolant enters the guide tube to flow upwardly therein, a portion of the coolant is allowed to escape the guide tube through a hole formed through the wall of the guide tube. In this regard, a prior art guide tube provides at least one generally rectangular hole of predetermined flow area that pierces the lower portion of the guide tube. A portion of the fluid admitted into the guide tube exits the guide tube through the hole. The remaining portion of the coolant flowing within the guide tube follows a flow path extending upwardly through the guide tube.
As the coolant is admitted into the prior art guide tube to flow upwardly therein, a portion of the coolant develops a cross flow component acting transversely to the control rods. This is so because, as the coolant flows upwardly within the guide tube, a portion of it will reverse or change direction (e.g., a change in direction of approximately 180.degree.) in order to flow out the hole piercing the lower portion of the guide tube. This portion of the coolant will flow out the hole because of a pressure gradient exiting between the interior and the exterior of the guide tube. However, applicant has observed that, as the previously mentioned portion of the coolant changes direction to flow out the hole, it will impinge the control rods at a velocity acting transversely thereto sufficient to cause the control rods to vibrate. This vibration is undesirable because such vibration may ultimately cause the control rods to wear against one or more of the guide plates through which they are slidably received. This wear may potentially compromise the ability of the control rods to suitably slide in the bores of the guide plates and thus may potentially compromise the ability of the control rods to suitably control the fission process. Therefore, a problem in the art is to reduce vibration of the control rods to prevent control rod wear.
Devices for reducing vibration of control rods are known. A single guide tube and tube sheet tube carrying a control rod having an inner tube mounted within the tube sheet tube is disclosed in U.S. Pat. No. 4,584,168 titled "System For Controlling Destructive Vibration Of A Nuclear Control Rod" issued Apr. 22, 1986 in the name of Frank J. Formanck. The inner tube disclosed by this patent is fitted closely about the control rod. Mounted connections between the upper and lower ends of the inner tube are provided to route coolant for purposes of controlling vibration of the control rod. Although this patent discloses a guide tube capable of controlling vibration of a control rod carried within it, this patent requires mounted connections to route the coolant. It appears that the mounted connections of the Formanck device may give rise to at least the possibility of loose parts migrating in the reactor vessel, which loose parts may damage the internal components in the reactor vessel, should the mounted connections become disassociated from the inner tube. This would be undesirable for safety reasons.
Therefore, what is needed is a control rod guide tube adapted to reduce vibration of a plurality of control rods disposed therein, which control rod guide tube is suitable for use in pressurized water nuclear reactors.